Activation systems in automotive applications include systems such as an airbag igniter system for airbag deployment and a seat belt pretensioner activation system.
An igniter system comprises an activation element or igniter element which converts electrical energy to heat. Typically, the igniter element, also known as a squib, comprises a hot wire bridge which is heated by a firing signal, for example a firing current of 1-2 Amps (A). In, for example, airbag applications, the heat generated in the igniter element ignites a pyrotechnic material adjacent the igniter element which burns a propellant. This produces gas to inflate the airbag.
A particular concern for automotive manufacturers is the possibility of activation elements activating inadvertently due to a fault. For example, inadvertent activation of an airbag may disturb a driver and possibly cause an accident. Thus, drive circuits used for generating the firing or activation signal are designed to minimise inadvertent activation and to ensure reliable operation. FIG. 1 illustrates a known simplified airbag activation circuit.
FIG. 1 illustrates an igniter element or squib 101 coupled to a drive circuit 103. The drive circuit 103 is implemented in a single Application Specific Integrated Circuit (ASIC) and comprises functionality for generating the firing signal which activates the squib 101. More specifically, the drive circuit 103 comprises a switch arrangement including a high side switch FET (Field Effect Transistor) 105 and a low side switch FET 107. During normal operation, when the airbag is not deployed, the high side FET 105 and the low side FET 107 are both in an off state and no current can flow through the squib. The use of two switch transistors in series provides increased reliability and failure prevention. Particularly, if either one of the switch FETs short circuits, this will not result in an activation of the airbag as the other switch FET will be in the off state.
The high side FET 105 and the low side FET 107 are each controlled by a control circuit 109. The control circuit 109 is coupled to a main processor 110 which is connected to one or more crash sensors, such as an accelerometer or acceleration sensor, only one 111 of which is shown in FIG. 1, to determine when a particular crash condition is occurring in which an airbag should be deployed. The control circuit 109 produces a signal which switches the low side FET 107 off during normal operation and on if the airbag is being activated and also controls the high side FET 105 to be off during normal operation and on during airbag activation.
Typically, the drive circuit 103 is not directly connected to the energy supply. Rather, a power switch transistor known as a safing switch 113 is coupled in series with the drive circuit 103. The safing switch 113 is generally an external discrete FET component. The safing switch 113 provides further failure prevention by providing additional redundancy in the airbag activation operation.
Specifically the operation of the safing switch 113 is controlled by a control circuit 115 in response to different sensor inputs than those used for activating the drive circuit. One safing acceleration sensor 112 is shown in FIG. 1. For semiconductor acceleration sensors, in order to enhance failure prevention, the safing switch 113 is controlled by a completely different microprocessor operating a different crash detection algorithm and with different sensor inputs than for the drive circuit 103. In this case, the control circuit 115 may be a small microprocessor. Thus, the airbag is only activated if both redundant evaluations detect the occurrence of a crash in which case the high side FET 105 and the low side FET 107 of the drive circuit as well as the safing switch 113 are switched on. The safing switch 113 is operated as a simple on/off switch. In some applications, several safing switches are used to provide independent safety switches for different drive circuits. For example, each squib may be provided with its own safing switch.
The safing switch 113 is coupled to a reverse flow blocking diode 117. The reverse flow blocking diode 117 is connected to a capacitor 125 coupled to receive the battery voltage Vbat and which provides the power supply to the drive circuit 103 and squib 101.
In the past, acceleration sensors 111 and 112 have typically comprised mechanical acceleration sensors. The output of each mechanical acceleration sensor is open or a short circuit depending on the sensed acceleration and controls the opening and closing of a transistor switch, such as the safing switch 113, the high side FET 105 and/or the low side FET 107. Thus, the output of the mechanical sensor determines whether the transistor switch is closed or open and thus, for example, whether the igniter element 101 is activated or not activated, respectively. Such mechanical sensors are large external components and thus increase the size and cost of an airbag system. In addition, since a spring in the mechanical sensor determines how long for which the transistor switch is closed, such a sensor does not allow you to vary how long the transistor switch is closed which may be desired on initial set-up and testing.
With the desire to reduce the size of airbag systems and provide a more programmable system, the safing mechanical sensor has been replaced by a small microprocessor and a micromachined acceleration sensor which has a linear output. The micromachined acceleration sensor is provided in a integrated package and is significantly smaller than the mechanical sensors. On a separate integrated circuit, a microprocessor, which in FIG. 1 is designated by reference numeral 115, processes the linear output from the micromachined sensor to control the opening and closing of safing switch 113. The microprocessor 115 may also coupled the linear output to the main airbag processor 110 for use in controlling the activation of the igniting element 101. Since the output of the micromachined acceleration sensor is linear and coupled to the microprocessor 115 such a system is much more flexible and allows for programming of parameters such as the acceleration value which causes the transistor switch to be closed.
Similarly, the mechanical acceleration sensors 111 coupled to the main processor 111 have been replaced by micromachined sensors and the functionality of the small microprocessor 115 has been incorporated into the main processor 110.
With the aim of reducing overall system cost, there is a desire to reduce the cost of implementing acceleration sensors in activation systems, such as airbag systems.